The present invention relates to wavelength-division-multiplexed, bidirectional optical fiber ring networks and, more particularly, to a system for accessing such bidirectional ring networks.
The ring is one of the most useful topologies for communication networks regardless of the underlying network technology. It is simple, easily managed, fault tolerant, and through interconnection, ring networks can serve as basic building blocks for more general network structures. Furthermore, because ring networks have been the preferred topology for “traditional” networks, such as SONET, they provide a natural evolutionary route to more advanced optical networks.
Because of the rapidly growing demand for bandwidth, wavelength division multiplexed (“WDM”) purely optical ring networks are currently the focus of much attention. An important need in the emerging WDM optical ring networks is for a cost effective technique for accessing such networks, i.e., connecting a network access station to the ring network. The cost of accessing the ring network includes the cost of the hardware in the ring network switching (add-drop) node, the cost of the fibers in the access link between the node and the network access station, and the cost of the optical transceivers in the network access station. Since this is a cost per network station access, minimization of this cost is important to the economic viability of large ring network architectures.
Referring to FIG. 1, there is shown a schematic diagram of a bidirectional ring network 100 having five network nodes 101 respectively connected to five network access stations (“NASs”) 102. The network nodes 101 of the ring network 100 are connected by internodal links, each consisting of a pair of optical fibers 103 and 104 carrying signals in opposite directions.
Turning now to FIG. 2, there is shown a schematic diagram of a known system for accessing a WDM bidirectional ring network in which each NAS 102 is connected to its respective network node 101 through two pairs of optical fibers 201 and 202, and 203 and 204. Because many wavelengths are multiplexed on each fiber, all wavelengths carried on a common fiber must be distinct to avoid interference. In the system of FIG. 2, a respective array of optical transmitters 214 and 216 and a respective array of optical receivers 213 and 215 is attached to each of the four access fibers 201, 202, 203 and 204, with the signals from the transmitter arrays 214 and 216 being “added” to the ring network via one of the two wavelength add drop multiplexers (“WADMs”) 207 and 208 in the network node 101, and the signals directed to the receiver arrays 213 and 215 being “dropped” from the ring network in a similar fashion.
The function of each one of the WADMs 207 and 208 is to selectively route signals passing through it according to their wavelengths. Each WADM has two states for each wavelength: the “bar” state, in which a signal having the wavelength entering at its upper input 205 or 219 leaves from its upper output 217 or 221, and a signal having the wavelength entering at its lower input 206 or 220 leaves from its lower output 218 or 222; and the “cross” state, in which a signal having the wavelength entering at its upper input 205 or 219 leaves from its lower output 218 or 222, and a signal having the wavelength entering at its lower input 206 or 220 leaves from its upper output 217 or 221. Accordingly, when both the first WADM 207 and the second WADM 208 of the network node 101 are each in the bar state for a given wavelength on the ring network, a signal having the given wavelength on the internodal fiber 209 entering the network node 101 leaves the network node through internodal fiber 210, and a signal having the given wavelength on internodal fiber 211 entering the network node leaves the network node on internodal fiber 212. If the first WADM 207 is in the bar state for a given wavelength and the second WADM 208 is in the cross state for the given wavelength, a signal having the given wavelength on internodal fiber 209 entering the network node 101 leaves the network node unchanged on internodal fiber 210. However, a signal having the given wavelength on internodal fiber 211 entering the network node is routed to the receiver array 213 through access fiber 201, thereby “dropping” the signal from the L fiber of the ring network, and a signal having the given wavelength provided by the transmitter array 216 through access fiber 204 is routed to the internodal fiber 212 leaving the network node, thereby “adding” the signal to the L fiber. If the first WADM 207 is in the cross state for a given wavelength and the second WADM 208 is in the bar state for the given wavelength, a signal having the given wavelength on internodal fiber 211 entering the network node 101 will pass through the network node unchanged to internodal fiber 212 leaving the network node. But a signal having the given wavelength on internodal fiber 209 entering the network node 101 is routed to receiver array 215 through access fiber 203, thereby dropping the signal routed to the receiver array 215 from the R fiber, and a signal having the given wavelength provided by the transmitter array 214 through access fiber 202 is carried on the internodal fiber 210 leaving the network node, thereby adding the signal provided by the transmitter array 214 to the R fiber.
It is noted that in the ring network access system illustrated in FIG. 2, each access fiber pair 201 and 204, and 202 and 203 connects a transmitter and a receiver to a WADM connected to one of the two unidirectional rings, so that the configuration is equivalent to two completely separate unidirectional ring networks, a clockwise ring (the R fiber) and a counterclockwise ring (the L fiber) with separate associated network access stations each having its own transmitter and receiver. The transmitter typically comprises an appropriate array of transmitting elements (e.g., semiconductor lasers and light emitting diodes) and the receiver typically comprises an appropriate array of receiving elements (e.g., photodiodes). Such a configuration is costly in terms of the number of transmitter arrays and receiver arrays required in the network access station, and in terms of the number of access fibers required to connect to the network node, particularly when the network access station 102 is located a considerable distance away from the network node 101. It would be highly desirable to have access systems for a bidirectional, wavelength-division-multiplexed optical fiber ring network that require fewer components and can therefore be implemented at lower cost.